Plant species associations in alpine-subnival vegetation patches in the Central Caucasus

Abstract. Patches of herbaceous vegetation found at the boundary of the alpine and subnival belts in the Central Caucasus (3000 m) were analysed for species composition, interspecific associations, as well as for relations between age, size and diversity of the patches. 34 plant species were recorded. Positive and negative associations were found among the most frequent species. A model is presented that describes the cumulation of species in patches during succession. In the final and stable successional stage, the maximum number of species within a patch was nine. Species composition varies in patches of different age, typical alpine species being more frequent in older ones. The results are interpreted as evidence for niche differentiation and selection of species composition through the 'sieve’ of interspecific relations, taking place during succession in the studied patches.     

Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation

Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi‐dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle‐bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores.     

Changes in individual plant traits and biomass allocation in alpine meadow with elevation variation on the Qinghai-Tibetan Plateau

Plant traits and individual plant biomass allocation of 57 perennial herbaceous species, belonging to three common functional groups (forbs, grasses and sedges) at subalpine (3700 m ASL), alpine (4300 m ASL) and subnival (⩾5000 m ASL) sites were examined to test the hypothesis that at high altitudes, plants reduce the proportion of aboveground parts and allocate more biomass to belowground parts, especially storage organs, as altitude increases, so as to geminate and resist environmental stress. However, results indicate that some divergence in biomass allocation exists among organs. With increasing altitude, the mean fractions of total biomass allocated to aboveground parts decreased. The mean fractions of total biomass allocation to storage organs at the subalpine site (7%±2% S.E.) were distinct from those at the alpine (23%±6%) and subnival (21%±6%) sites, while the proportions of green leaves at all altitudes remained almost constant. At 4300 m and 5000 m, the mean fractions of flower stems decreased by 45% and 41%, respectively, while fine roots increased by 86% and 102%, respectively. Specific leaf areas and leaf areas of forbs and grasses deceased with rising elevation, while sedges showed opposite trends. For all three functional groups, leaf area ratio and leaf area root mass ratio decreased, while fine root biomass increased at higher altitudes. Biomass allocation patterns of alpine plants were characterized by a reduction in aboveground reproductive organs and enlargement of fine roots, while the proportion of leaves remained stable. It was beneficial for high altitude plants to compensate carbon gain and nutrient uptake under low temperature and limited nutrients by stabilizing biomass investment to photosynthetic structures and increasing the absorption surface area of fine roots. In contrast to forbs and grasses that had high mycorrhizal infection, sedges had higher single leaf area and more root fraction, especially fine roots.     

Timing of sexual reproduction and reproductive success in the high-mountain plant Saxifraga bryoides L

SAXIFRAGA BRYOIDES L. is one of the plant species reaching the upper limits of distribution for flowering plants in the European Alps. Because of its abundance in the subnival and nival zones, we expected S. BRYOIDES to reproduce efficiently in the highly stochastic climate at higher altitudes. Investigations were carried out at two subnival sites (2650 m and 2880 m a.s.l.) in the Austrian Alps. We studied flowering phenology, dynamics of seed development, and reproductive success in the climatically different years from 2001 - 2004. For a nival plant species, S. BRYOIDES showed a particularly long prefloration period (6 - 9 weeks). From onset of anthesis until seed maturity took an individual flower another 6 - 7 weeks and all individuals at a site 9 - 10 weeks. The length of the prefloration period and seed histogenesis was temperature-dependent, whereas seed maturation seemed to be endogenously controlled. Only in the exceptionally long and warm growing season of 2003 did all fruits mature at a site. In the other years, the onset of winter conditions halted development in many fruits before maturity. The seed/ovule ratio of mature fruits was around 0.7 in all years. The relative reproductive success (RRS) ranged from zero to 0.7, depending on the site and year. In conclusion, S. BRYOIDES needs an unexpectedly long time to undergo reproductive development. Though fruit maturation is uncertain, the high S/O ratio of single intact fruits results in at least a small seed crop in most years. This seems to be sufficient to assure the spread and maintenance of S. BRYOIDES at higher altitudes. As a seed-risk strategist (Molau,1993), S. BRYOIDES would clearly benefit from a prolonged growing season, which might occur more often if climate warming continues.     

Foliar nitrogen and phosphorus stoichiometry of alien invasive plants and co-occurring natives in Xishuangbanna

Aims How alien invasive plants and co-occurring native plants utilize nutrients is one of major issues in invasion ecology. Foliar nitrogen (N) and phosphorus (P) contents and stoichiometry can elucidate the uptake ability and limitation status of nutrients in plants, which provides basic knowledge for understanding the invading ability and co-occurrence or disappearance of plants.
Methods Based on typical alien invasive plants (Chromolaena odorata, Ageratina adenophora) and native plants in southwestern China, this study focused on strategies of N and P utilization among invasive plants and native plants under different invasion conditions. The species compositions, aboveground biomass, leaf N and P contents and leaf N:P were investigated for plants in plots with no invasion and with different invasion extents (estimated by the plot-based percentage of invaders’ biomass in total community) at Mt. Kongming in Xishuangbanna region, Yunnan Province, China.
Important findings The species number decreased significantly with the invasion extent of both C. odorata and A. adenophora, although the aboveground biomass was greatly enhanced. Leaf N and P contents did not differ between the two studied invaders, but they showed significantly higher N and P levels than both co-occurring and only native species (p < 0.05). Besides, leaf N and P contents of invaders increased with the invasion extent, and leaf N of native plants also showed an increasing trend with the invasion extent. When the influence of invasion was checked for the same species, leaf P contents decreased, whereas leaf N and N:P increased for most native plants under invasion. Based on the absolute foliar N and P contents, N:P values, we inferred that native plants were still limited by N, although N availability might be enhanced by invasion. Both invasive plants had leaf N:P values lower than 10, suggesting a higher P uptake relative to N uptake. All above results highlighted a higher N and P uptake of typical alien invasive plants in southwestern China.     

神农架地区典型草本群落中的昆虫访花行为比较

对神农架地区不同海拔高度(1,000–2,600 m)的典型草本群落中的昆虫访花行为进行了初步的监测和统计。在9个样地中共观察到鳞翅目、膜翅目、双翅目11科昆虫访花, 访花者的类群在不同海拔高度的群落中有所不同, 但明显与植物群落的物种组成有关。对昆虫访花频率的监测发现, 不同物种组成的草本群落中, 所有昆虫访花的总频率和不同类群的昆虫访花频率都有差异, 正在开花的物种较多的群落中通常能吸引相对较多的访花者。同一种植物在不同的群落背景中受访频率差异显著, 如花被不甚显著的酸模(Rumex acetosa)在某些群落中未观察到访花者, 在另一些群落中却有昆虫频繁访问。而花被显著的血见愁老鹳草(Geranium henryi)在各群落中都有频繁的昆虫访问。一些植物(如广布野豌豆Vicia cracca)虽然在不同群落中都有昆虫访问, 但访花者类群却有较大差异。作者认为, 在植物群落中, 不同植物的花在吸引昆虫传粉方面存在着一定的相互作用, 在研究特定植物的虫媒传粉时应重视该植物种群所处的群落背景所起的作用。     

西双版纳外来入侵植物及其共存种叶片氮、磷化学计量特征

诊断外来植物和本地种的资源利用方式是入侵生态学研究的热点问题。叶片氮(N)、磷(P)含量和化学计量特征可以反映受侵地区植物的N、P吸收能力和限制状况, 为把握外来植物的入侵能力、为本地植物共存或消失的机制提供基础科学依据。该研究以我国西南地区典型外来入侵植物(飞机草(Chromolaena odorata)、紫茎泽兰(又叫破坏草, Ageratina adenophora))及其共存的本地植物为对象, 探讨不同入侵条件下入侵种及其共存的本地植物的N、P利用策略。在云南西双版纳孔明山研究区, 调查了无入侵条件下和飞机草与紫茎泽兰的不同入侵程度(按入侵种生物量比例划分)下的物种数量和生物量, 分析了主要植物的叶片N、P含量和N:P值。结果显示: 尽管群落地上生物量随飞机草和紫茎泽兰入侵程度增加而增加, 但本地植物种数随飞机草和紫茎泽兰入侵程度增加而显著减少。飞机草与紫茎泽兰的叶片N、P平均含量无显著差异, 但均显著高于无入侵条件下的本地植物以及与其共存的本地植物。两种入侵植物的N、P含量均随它们所占样方总生物量的比例增大而升高, 本地植物N含量也有相似的变化趋势。当对比入侵和无入侵两类样方的同种植物N、P和N:P变化时发现多数本地种叶片P含量呈降低趋势, N含量和N:P呈升高趋势。根据叶片N、P绝对含量和N:P值及其随入侵的变化规律, 推测入侵可能提高了植物的N可利用性, 但本地植物仍然受N限制; 入侵植物N:P值总体小于10与其具有相对于N吸收的较高的P吸收能力有关。该研究揭示了西南典型外来入侵植物具有较强的N、P吸收富集能力。     

Competitive effects of non-native plants are lowest in native plant communities that are most vulnerable to invasion

Despite widespread acknowledgment that disturbance favors invasion, a hypothesis that has received little attention is whether non-native invaders have greater competitive effects on native plants in undisturbed habitats than in disturbed habitats. This hypothesis derives from the assumption that competitive interactions are more persistent in habitats that have not been recently disturbed. Another hypothesis that has received little attention is whether the effects of non-native plants on native plants vary among habitats that differ in soil fertility. We documented habitat occurrences of 27 non-native plant species and 377 native plant species encountered in numerous study plots in a broad sample of ecosystems in MS (USA). We then reviewed experimental and regression-based field studies in the scientific literature that specifically examined potential competitive (or facilitative) effects of these non-native species on native species and characterized the habitats in which effects were the greatest. As expected, the non-native species examined here in general were more likely to be associated with severely disturbed habitats than were the native species as a group. In contrast, we found that non-native species with competitive effects on natives were more likely to be associated with undisturbed habitats than with disturbed habitats. When longer term studies involving more resident species were given more weight in the analysis, competitive effects appeared to be the greatest in undisturbed habitats with low soil fertility. These results reinforce the notion that invasion is not synonymous with impact. The environmental conditions that promote invasion may limit competitive effects of invaders on native plant communities following invasion.     

How landscape modulators function: woody plant impact on seed dispersal and abiotic filtering

The Mediterranean landscape is characterized by a heterogeneous structure: a mosaic of woody plants (trees or shrubs) with scattered patches of herbaceous vegetation. Although the herbaceous and woody patches are adjacent to each other, plant species composition in them is substantially different. This could be attributed to either differences in environmental conditions between patch types (i.e., abiotic filters), or to dispersal limitations caused by the woody plants acting as dispersal filters. In this article, we focus on the relative impact of woody plants, applying these two filter types, in determining plant species composition in Mediterranean woodland. We experimentally manipulated shade and litter cover and examined the effect of each of these factors on plant species composition. We used seed-traps to evaluate seed arrival in the patches, and experimentally removed the shrub canopy to study the effect of the shrub as a physical barrier to seed entry. Results showed that plant species number and composition were not significantly affected by shade and litter manipulation. The number of trapped seeds were significantly higher in the open patches than in the woody patches, and removal of woody plants increased the number of trapped seeds in both open and woody patches, as a result of eliminating the physical obstacle to free seed movement. Our findings show that woody plants affect the herbaceous plant community by influencing seed dispersal, and highlight that they affect other organisms not only by modifying resource availability but also through the creation of a new landscape structure.     

The role of plant species size in invasibility: a field experiment

Large plant species self-thin to disproportionately lower densities than smaller plant species, and therefore may leave more patches of unused space suitable for invasion. Using experimental monocultures of 11 old-field perennial plant species differing in maximum size, as well as mixtures composed of all monoculture species, we tested our primary hypothesis that monocultures of larger species will be more susceptible to natural invasion. After 3 years, monocultures of larger species were invaded by a significantly greater number of species, and more ramets, from the surrounding vegetation. Invading plant species were significantly smaller than the monoculture species being invaded, suggesting that smaller plant species may be better invaders. Thus, we quantified a trade-off between species size, which is frequently associated with increased competitive ability for light, and invasibility, suggesting one reason why large and small species coexist in virtually all plant communities. Although we expected that invasion would enhance biomass production by more fully capturing available resources, we found that the most highly invaded plots of each species produced significantly less biomass. This suggests that increased diversity resulting from invasion did not result in complementary resource use. Mixture plots containing all experimental species did not admit a significantly different number of invading ramets or species than most monocultures, indicating no obvious role for diversity in resistance to invasion, or complementary resource use. Our results suggest that relatively large species may be limited in their capacity to competitively exclude other, smaller species from communities because pure stands of the former are more susceptible to invasion by the latter.     

Population ecology of wild sunflowers: effects of seed density and post-dispersal vertebrate seed predators

Assessing the effects of seed density on the population dynamics of wild plant species with crop relatives will be vital in determining the potential effects of introducing traits into wild populations as a result of crop-to-wild gene flow. We examined experimental sunflower (Helianthus annuus) patches in eastern Kansas to determine the effects of seed density and predation on seedling recruitment and seed production in the next generation. High seed density treatment plots had significantly more seedlings and adult plants than did low seed density treatment plots. Overwinter vertebrate seed predator exclusion treatments resulted in increases in plant density compared to plots in which vertebrates were not excluded. Control patches (no seeds added) contained virtually no plants. Head production and estimated total seed production for a patch were not statistically different among treatments (excluding control plots). Although initial seed density and vertebrate post-dispersal seed predation do appear to have effects on seedling recruitment, neither appear to be limiting seed production of competing adult plants. Therefore, variation in seed densities (over the range examined) may have limited effects on local population dynamics. It is important to note that the choice of seed densities may affect the results obtained: the seed densities used in this study may, in retrospect, be higher than in the small roadside populations typical in eastern Kansas, yet other natural sites have much larger densities. Further, the effects of increased seed density at a local site may have other important effects such as altering metapopulation dynamics through increased long-distance dispersal or increased local seed bank size.     

Can more K‐selected species be better invaders? A case study of fruit flies in La Réunion

Invasive species are often said to be r -selected. However, invaders must sometimes compete with related resident species. In this case invaders should present combinations of life-history traits that give them higher competitive ability than residents, even at the expense of lower colonization ability. We test this prediction by comparing life-history traits among four fruit fly species, one endemic and three successive invaders, in La Réunion Island. Recent invaders tend to produce fewer, but larger, juveniles, delay the onset but increase the duration of reproduction, survive longer, and senesce more slowly than earlier ones. These traits are associated with higher ranks in a competitive hierarchy established in a previous study. However, the endemic species, now nearly extinct in the island, is inferior to the other three with respect to both competition and colonization traits, violating the trade-off assumption. Our results overall suggest that the key traits for invasion in this system were those that favoured competition rather than colonization.     

Dry matter partitioning and root length/leaf area ratios in herbaceous perennial plants with diverse altitudinal distribution

Summary Partitioning patterns in 22 exclusively low and 27 exclusively high altitude perennial herbaceous species were examined in order to test the hypothesis that plants of high altitudes allocate more dry matter to below-ground parts and in particular to storage organs, than typical low altitude plants. Our results raise some doubts about the general validity of this hypothesis. The mean fractions of total dry matter allocated to green leaves (22±2% s.e. at low and 24±2% at high altitude) and special storage organs (28±4% at both altitudes) do not differ significantly among sites. The mean relative portions of total dry matter allocated to above-ground plant parts amount to 57±3% at low and 42±3% at high elevation (P=0.002) and differ less than often assumed. The greater below-ground fraction at high altitude results from reduced stem and proportionally increased fine root compartments. At high altitude specific root length is increased by 50% and mean individual rooting density is tripled. Fine root length per unit leaf area is 4.5 times greater (P<0.001). However, interspecific variation in all these quantities is considerable and species with quite contrasting partitioning patterns coexist at both elevations. This suggests that the success of perennial herbaceous plants at high elevations does not necessarily depend on a large below ground biomass fraction. The increased fine root length at high altitude may substitute for reduced mycorrhizal infection. Figure 1 provides a graphical summary.     

Short-term signals of climate change along an altitudinal gradient in the South Alps

Short-term changes in plant species number, frequency and composition were studied along an altitudinal gradient crossing four summits from the treeline ecotone to the subnival zone in the South Alps (Dolomites, Italy). Large-scale (summit areas) and small-scale patterns (16 plots of 1 m2/summit) were monitored. After 5 years, a re-visitation of the summit areas revealed a considerable increase of species richness at the upper alpine and subnival zone (10% and 9%, respectively) and relatively modest increases at the lower alpine zone and the treeline ecotone (3% and 1%, respectively). At the small scale, the results were partly different, with species richness decreasing at the lower summits and increasing at the higher summits. The changes can most likely be attributed to climate warming effects and to competitive interactions. The main newcomers at the lower three summits were species from the treeline and the lower altitudinal zones. Only at the highest summit, the newcomers came from the alpine species pool. At the treeline ecotone, the abundance of Pinus cembra, of dwarf shrubs and clonal graminoid species increased. Here, displacements of alpine species may be predicted for the near future. At the higher summits, expansions of the established alpine species and further invasions of species from lower altitudes are forecasted.     

Determinants of the geographical extent of invasive plants in China: effects of biogeographical origin,life cycle and time since introduction

Plant invasions cause serious impacts to environment and human economy, and lots of researches have been paid to examine factors that determine the geographical extent of plant invaders. Many intrinsic species attributes, external environmental factors, and time since introduction have all been proposed to explain invasive success. Data on invasive plants distribution in province regions in China were collected, and species attributes and historical factors such as biogeographical origin, life cycle, and time since introduction were determined. While some annuals may eventually occupy the whole country area, perennials are unlikely to do so, particularly those from Central and South America. Time since introduction is significantly related to the number of provinces occupied by an invader; nevertheless after control of certain residence time, annuals are not likely to more provinces than the perennial herbs. High-impact invaders tend to spread faster than other invaders, but only when their life cycle is taken into account. This suggests that other plant traits, instead of life cycle, along with extrinsic factors, greatly contribute to invasive success and distribution area. Invasive plants in China are mainly distributed in human-made habitats, and their long-distance dispersal is primarily associated with human activities. Future studies need to consider interactions between species traits, environmental conditions, and human activities for further insights in researches alike.     

Clonal splitters and integrators in harsh environments of the Trans-Himalaya

Individuals of clonal plants consist of physically and physiologically connected ramets. In splitters, they are integrated for a time shorter than ramet generation time (i.e. the time it takes to produce the first offspring ramet), whereas in integrators connections between ramets persist for a longer time. It has been predicted that integrators should prevail in stressful environments, such as habitats poor in nutrients, whereas splitters are expected to dominate in benign habitats, such as fertile areas with a moderate climate. I tested these predictions in four dry mountain areas of the Trans-Himalaya, in high altitudes subjected to multiple stresses. In accordance with the expectations I found that clonal plants with integrated ramets reach higher mean and maximum altitudes than splitters. Integrators were over-represented in nutrient-poor habitats, such as dry semi-deserts, sandy steppes and in subnival habitats, whereas splitters preferentially colonised mesic habitats, saline sites and wetlands. While there was no difference in the representation of splitters and integrators in habitats with an unstable surface, such as screes, dunes and water bodies, fully integrated clonal plants preferred very stable environments, such as banks of streams covered by closed-canopy vegetation. Most relationships between clonal integration and environmental factors were explainable by the phylogenetic relationship between the species, only the significant preference of splitters for shaded environments persisted in phylogenetically corrected analysis. The results indicate that clonal integration belongs to a set of evolutionarily conservative plant traits, usually shared by related species. Consequently, the adaptive value of clonal integration in individual habitats remains questionable.     

The Root-Associated Microbial Community of the World’s Highest Growing Vascular Plants

Upward migration of plants to barren subnival areas is occurring worldwide due to raising ambient temperatures and glacial recession. In summer 2012, the presence of six vascular plants, growing in a single patch, was recorded at an unprecedented elevation of 6150 m.a.s.l. close to the summit of Mount Shukule II in the Western Himalayas (Ladakh, India). Whilst showing multiple signs of stress, all plants have managed to establish stable growth and persist for several years. To learn about the role of microbes in the process of plant upward migration, we analysed the root-associated microbial community of the plants (three individuals from each) using microscopy and tagged amplicon sequencing. No mycorrhizae were found on the roots, implying they are of little importance to the establishment and early growth of the plants. However, all roots were associated with a complex bacterial community, with richness and diversity estimates similar or even higher than the surrounding bare soil. Both soil and root-associated communities were dominated by members of the orders Sphingomonadales and Sphingobacteriales, which are typical for hot desert soils, but were different from communities of temperate subnival soils and typical rhizosphere communities. Despite taxonomic similarity on the order level, the plants harboured a unique set of highly dominant operational taxonomic units which were not found in the bare soil. These bacteria have been likely transported with the dispersing seeds and became part of the root-associated community following germination. The results indicate that developing soils act not only as a source of inoculation to plant roots but also possibly as a sink for plant-associated bacteria.     

Impacts of exotic invertebrates on New Zealand?s indigenous species and ecosystems

Biological invasions have significantly affected New Zealand?s native species and ecosystems. Most prominent are the effects of exotic mammals and plants, whereas few invertebrate invasions are known to have major effects on native ecosystems. Exceptions are the well-known cases of Vespula wasps in Nothofagus forest ecosystems and Eriococcus scale insects in Leptospermum shrublands. This limited impact is surprising because over 2000 exotic invertebrates have become established in New Zealand, among them many pests of exotic crop plants. The low impact of exotic invertebrates that invaded forests and other native ecosystems in New Zealand is in contrast to the situation in other parts of the world where many invertebrates have become important pests. We provide an overview of known invasions by exotic invertebrates in New Zealand, and explore in more detail several examples of invasive species, including herbivores, predators, parasitoids, decomposers and other groups in forests, grasslands, and other terrestrial ecosystems. Several hypotheses have been proposed to explain the comparative scarcity of such invasions that affect New Zealand?s indigenous ecosystems. There is a common view that New Zealand?s native species and ecosystems are inherently resistant to exotic invertebrate invaders, and there is some evidence to support this view. As a result of the high level of endemism in New Zealand?s flora, many native plants are phylogenetically distant from the host plants of many plant-feeding invaders. This provides some protection. Less host-specific plant-feeding insects, generalist predators, parasitoids and decomposers are less affected by such constraints, and these groups are perhaps more represented among the successful invaders of natural ecosystems. However, the shortage of studies on invader impacts on native species and ecosystems, compared with studies on economically important crops and production ecosystems, means that an unbiased comparison is not possible at this time. Furthermore, many invaders go through extended lag phases where their impacts are not easily noticed until they become more abundant and create more damage. Likewise, indirect effects of invaders, through more complex interactions in food webs, as well as impacts on ecosystem functions such as decomposition and pollination, are more subtle and difficult to detect without careful study. There is clearly a need for more research to determine more accurately which exotic invertebrates are already present, what their direct and indirect impacts are, and what generalisations and predictions about threats to native species and ecosystems are possible.     

Testing the Stress-Gradient Hypothesis at the Roof of the World: Effects of the Cushion Plant Thylacospermum caespitosum on Species Assemblages

Many cushion plants ameliorate the harsh environment they inhabit in alpine ecosystems and act as nurse plants, with significantly more species growing within their canopy than outside. These facilitative interactions seem to increase with the abiotic stress, thus supporting the stress-gradient hypothesis. We tested this prediction by exploring the association pattern of vascular plants with the dominant cushion plant Thylacospermum caespitosum (Caryophyllaceae) in the arid Trans-Himalaya, where vascular plants occur at one of the highest worldwide elevational limits. We compared plant composition between 1112 pair-plots placed both inside cushions and in surrounding open areas, in communities from cold steppes to subnival zones along two elevational gradients (East Karakoram: 4850–5250 m and Little Tibet: 5350–5850 m). We used PERMANOVA to assess differences in species composition, Friedman-based permutation tests to determine individual species habitat preferences, species-area curves to assess whether interactions are size-dependent and competitive intensity and importance indices to evaluate plant-plant interactions. No indications for net facilitation were found along the elevation gradients. The open areas were not only richer in species, but not a single species preferred to grow exclusively inside cushions, while 39–60% of 56 species detected had a significant preference for the habitat outside cushions. Across the entire elevation range of T. caespitosum, the number and abundance of species were greater outside cushions, suggesting that competitive rather than facilitative interactions prevail. This was supported by lower soil nutrient contents inside cushions, indicating a resource preemption, and little thermal amelioration at the extreme end of the elevational gradient. We attribute the negative associations to competition for limited resources, a strong environmental filter in arid high-mountain environment selecting the stress-tolerant species that do not rely on help from other plants during their life cycle and to the fact the cushions do not provide a better microhabitat to grow in.     

Invasion of pollination networks on oceanic islands: importance of invader complexes and endemic super generalists

Abstract. The structure of pollination networks is described for two oceanic islands, the Azorean Flores and the Mauritian Ile aux Aigrettes. At each island site, all interactions between endemic, non-endemic native and introduced plants and pollinators were mapped. Linkage level, i.e. number of species interactions per species, was significantly higher for endemic species than for non-endemic native and introduced species. Linkage levels of the two latter categories were similar. Nine types of interaction may be recognized among endemic, non-endemic native and introduced plants and pollinators. Similar types had similar frequencies in the two networks. Specifically, we looked for the presence of 'invader complexes' of mutualists, defined as groups of introduced species interacting more with each other than expected by chance and thus facilitating each other's establishment. On both islands, observed frequencies of interactions between native (endemic and non-endemic) and introduced pollinators and plants differed from random. Introduced pollinators and plants interacted less than expected by chance. Thus, the data did not support the existence of invader complexes. Instead, our study suggested that endemic super-generalist species, i.e. pollinators or plant species with a very wide pollination niche, include new invaders in their set of food plants or pollinators and thereby improve establishment success of the invaders. Reviewing other studies, super generalists seem to be a widespread island phenomenon, i.e. island pollination networks include one or a few species with a very high generalization level compared to co-occurring species. Low density of island species may lead to low interspecific competition, high abundance and ultimately wide niches and super generalization.